As Harvey continues to pound Houston, America’s fourth largest city, with rainfall for the sixth day in a row, the waters are still rising and emergency workers are still rescuing victims of the storm. Many have lost their homes and nearly everything they own. According to Houston Mayor Sylvester Turner, the focus will continue to be on rescue rather than damage assessment or recovery. Officials have warned that conditions may not improve for some time, with governor Greg Abbott warning residents of southeast Texas to brace for “a new normal.”
As of Tuesday, the storm has set a new record for total rainfall from a single tropical storm in the continental US, according to the National Weather Service. Two weather stations in the area have reported total rainfall of more than 48 inches. Local officials have reported 13 deaths related to the storm. The Houston Police Department has alone rescued 3,500 people from flooding since the start of the storm late last week. Rain is expected to continue as late as Friday of this week.
The question on many minds in the wake of such a catastrophic storm is the role that global climate change, a result of human activities, may have played in the severity of the storm. The short answer, according to scientists so far, is that climate change has worsened the effects of Harvey in some measurable, provable ways. Hurricanes, of course, are nothing new for the Gulf region, but the catastrophic effects of storms in recent years can be traced to changes tied directly to climate change. Storms such as Harvey are an important reason to aggressively combat the effects of climate change, and to strive to achieve the goals laid out in the Paris agreement – precisely the opposite of the Trump administration’s approach to the issue so far.
According to a statement from the World Meteorological Association issued Tuesday, “Climate change means that when we do have an event like Harvey, the rainfall amounts are likely to be higher than they would have been otherwise.”
For one thing, warmer seas evaporate more quickly – warmer air holds more water vapor. With temperatures around the world indisputably on the rise, the air stores more moisture which results in more intense precipitation. The Clausius-Clapeyron equation shows that every half degree Celsius of warming results in roughly a 3 percent rise in moisture stored in the atmosphere. In the Gulf of Mexico, surface temperatures are more than half a degree Celsius higher than recent averages for this time of year, which is itself over half a degree higher than averages 3 decades ago, according Penn State University Professor of atmospheric science, Michael E. Mann. These higher surface temperatures result in a higher potential for precipitation.
There is no room for doubt that this is occurring. The levels of precipitation during Harvey are unprecedented in Southeast Texas. The National Weather Service was forced to add a new color on its graphs to represent new levels of precipitation. On Tuesday, the previous record for rainfall from a tropical storm in the US was broken when 49.2 inches was recorded in Southeast Houston. This combination of coastal flooding and heavy precipitation accounts for the level of catastrophic flooding that Houston is experiencing.
Warmer temperatures are not the only link between the severity of Harvey and global climate change. Sea levels have risen 20 centimeters as a direct result of human activities over 100 years. Glaciers have melted and pushed sea levels higher – this resulted in a greater storm surge, and more flooding, during Harvey.
These verified connections between Harvey and climate change leave little room for argument. In addition to these proven links, however, there are also new questions raised by Harvey that scientists are working to answer. According to University of Oxford Royal Society Research Professor Tim Palmer, the most important of these involves the fact that Harvey remained stationary above Houston for an unusual period, worsening the precipitation and resulting damage during the storm. While there is no clear link as of yet between this phenomenon and climate change, scientists have noticed a broad slowing of atmospheric summer circulation in mid-latitudes, connected to strong warming trends in the Arctic.
According to Stefan Rahmstorf, co-chair at the Potsdam Institute for Climate Impact Research:
“This can make weather systems move less and stay longer in a given location – which can significantly enhance the impacts of rainfall extremes, just like we’re sadly witnessing in Houston.”
The still budding field of attribution science has made great strides in recent years, seeking to prove whether specific extreme weather events can be attributed to climate change. As the dust settles from Harvey, scientists may be able to use computer models, temperature records, and emissions statistics to do the math on Hurricane Harvey. There is no doubt however, that the storm’s behavior suggests a connection to global climate change. The catastrophic nature of Hurricane Harvey, and the potential for more storms like this in the future, should factor in to policy decisions regarding climate change.